This invention relates to the art of trapped rubber molding and, more particularly, to fixed-volume trapped rubber molding.
Many modern-day structural components are made from materials such as fiberglass or graphite. Such components, typically, are composed of multiple layers of so-called pre-pregs (combination resin and fiber materials) which have been laid up over a die and thereafter cured under rather precise temperature and pressure conditions The curing of the pre-preg layup, in most instances, is accomplished in an autoclave and requires the use of a vacuum bag or other type of barrier to prevent the pressurized fluid from penetrating the layers and, thus, ruining the component which is being fabricated.
Autoclaves which are suitable for forming such components, as will be readily appreciated by those skilled in the art, are extremely expensive. The vacuum bags, moreover, have a marked tendency to leak, resulting in a high reject rate. A need has existed for a substantial period of time for a process and apparatus wherein the layups for components of this type can be cured or molded under rather precise temperature and pressure conditions, which do not require the use of vacuum bags or other types of barriers and which reduce the incidence of unacceptable finished components to an acceptable level.
It has been proposed heretofore to utilize trapped rubber molding systems to cure such components. These systems, basically, involve the use of a closed container or mold within which a cured elastomer such as silicone rubber having a high coefficient of thermal expansion is placed. The heating of the part on the die in the container causes the elastomer to expand and, when the apparatus is properly configured, applies a uniform isostatic pressure to the pre-preg layup on the die.
These previously proposed trapped rubber molding systems have been of both the variable and fixed-volume types. The variable volume type is illustrated, for example, in U.S. Pat. No. 4,624,820, issued Nov. 26, 1986, and entitled "Molding of Composite Materials." These systems, in general, utilize an external source of hydraulic pressure-i.e., a piston or plate--to close and pressurize the interior of the container during the initial phases of the molding process. As the elastomer is heated and expands, the pressure exerted by the external source is reduced, effectively increasing the volume of the container to maintain the pressure within desired limits.
The fixed-volume type of trapped rubber molding system is illustrated, for example, in U.S. Pat. No. 4,167,430, issued Sept. 11, 1979, and entitled "Method for Fabricating a Composite Bonded Structure." This type of system, to which the present invention relates, utilizes a fixed-volume, rigid container or mold within which the die, the part to be molded and the elastomer are positioned.
The fixed-volume type of trapped rubber molding system is extremely simple from a mechanical standpoint requiring, in effect, no moving parts. Pressures, however, tend to exceed desired and, sometimes, safe limits. If, for example, the elastomer in the fixed-volume container has been cured at or near room temperature and if, when the mold and layup are assembled, the elastomer substantially fills all voids, pressures, upon application of heat, get totally out of hand. It has been previously proposed, thus, to form the elastomer such that, at the commencement of the molding process, it does not fill the mold. One proposal has been to merely leave a gap or a so-called ullage within the mold such that it does not completely fill and thereafter pressurize the laid-up part until a specified temperature has been reached. A similar prior proposal involves curing of the elastomer within the mold at an elevated temperature, thus achieving similar results. While procedures such as these have resulted in controllable pressures in certain uses, they do not permit independent control of pressure and temperature. Particularly, they do not permit maintaining pressure upon the near-finished component as it is cooled below the curing temperature.
Nor do these procedures permit repeated use of a particular mold. Each time the mold is cycled the elastomer which it contains changes. What was the proper ullage for the first cycle will not be the proper ullage for the second cycle. The same for the second to third, and so on. The result, thus, is that even if a given ullage gives the desired temperature-pressure relationship during the first cycle, it may not do so for repeated cycles thereafter.
One potential solution to these problems, as recognized in the prior art, is to selectively cool a portion of the elastomer within the container remote from the die. Elastomers of the types used in those environments have a relatively low coefficient of thermal conductivity and, assuming a relatively constant coefficient of thermal expansion through a given temperature range, pressures should be controllable by causing one portion of the elastomer within the mold to shrink while that portion adjacent the die expands as the desired temperature at the die interface is reached. This solution has been utilized with some success in so-called "crumb" rubber systems where the elastomer is particulate or pulverized and pumped into and out of a pressurized mold each time a cycle is run. Such systems, however, require the use of a vacuum bag, foil or similar barrier system over the laid-up part to be molded. They also require expensive and complicated pumps, pressure vessels and the like.
No one, to Applicant's knowledge, has heretofore successfully applied this theory to a fixed-volume mold containing an elastomer which is an integral solid--i.e., a noncrumb, de-aired elastomer. No one, specifically, has devised a system and method of using the same wherein a part of the type under consideration can be molded under relatively precise temperature and pressure conditions without the use of a vacuum bag or similar barrier mechanism and wherein the same mold can be used repeatedly for production of a rather large number of parts with an insignificant number of rejects. It is an object of this invention to provide such a system and method of using the same.